FUTURE generations should live longer, perhaps even to more than 150 years old, a leading expert on ageing will predict today at the start of Britain's biggest annual science gathering.

Professor Grimley Evans, head of Oxford University's division of gerontology, will tell the annual meeting of the British Association for the Advancement of Science how science and medicine have already found ways to increase average lifespan. He is now confident that today's work on genetic techniques promises to lengthen maximum lifespan too.

At present, the maximum human lifespan is about 115 years. In his speech, Professor Evans contrasts this figure with average lifespan, which varies considerably. Economically developed countries score well, with Japan and Iceland at the top of the league.

Differences in behaviour are known to affect average lifespans - giving up smoking, cutting down alcohol and sensible diet and exercise regimes are all important. 'There are still years to be gained from changes in lifestyle and environment,' Professor Evans argues.

But ageing is the result of complicated interactions between a person's environment and the genes in their body. Once people reach their mid-20s, death rates rise continuously. Elderly people should not, therefore, be 'separated off' from the rest of the human race 'unless we define the elderly as comprising everyone over the age of 13', Professor Evans says.

He suggests that there are no grounds for assuming that prolonging healthy life will increase human suffering, or medical costs to society. In his keynote address on prolonging life, Professor Evans suggests three approaches to push up the 115-year maximum.

The first would involve breeding selectively from the frozen sperm of men who die old; the second would be to modify the genes responsible for ageing; and the third would be to interfere with the biological processes that harm the body's cells. The aim would be to take the strain off the body's natural mechanisms for preventing, spotting and repairing damage.

But Professor Evans's speech at the week-long conference at Keele University also sounds a few warning notes. Scientists have little idea if their techniques for helping the body to survive would be equally effective on all organs. Neither is it clear what effect postponing death might have on disorders associated with old age, such as Alzheimer's Disease. 'Postponing death without postponing Alzheimer changes in the brain, or the onset of stroke, might not be a good investment either for the individual or for society,' Professor Evans concludes.